One of the problems involved in the detection and measurement of small traces of oil in water using a light scattering technique is that of differentiating between oil droplets and suspended solid particles. This problem is particularly acute in a marine environment where water almost invariably contains particles of rust released from ferrous metal surfaces. In a conventional oil-in-water detector the intensity of light scattered at an angle to an incident light beam is measured and an oil level reading is derived directly from this intensity. It will be appreciated that the presence of suspended solid particles in the water can, at low oil levels, lead to widely inaccurate results. In an attempt to overcome this problem the use of a plurality of photodetectors each arranged at a different scattering angle to an incident light beam has been proposed. Since the angular scattering characteristics of oils and solids are different it is possible to extract from the outputs of the various detectors a measure of the oil level. Such an arrangement however is relatively costly and cannot readily be fitted to those existing installations which employ a single scattering angle for the detection process.
The object of the invention is to minimize or to overcome this disadvantage.